1. Field of the Invention
The present invention relates to a video encoder including a signal combining circuit for generating a composite video signal by combining a luminance signal and color signal and an image processing system using the same.
2. Description of the Related Art
In general, a video encoder uses a signal combining circuit shown in FIG. 23 in order to generate a composite video signal including a luminance signal Y and a color signal C.
This combining circuit mixes the luminance signal Y input from a terminal T1 and the color signal C input from a terminal T2 by a mixer 1 to generate a composite video signal Sc.
FIGS. 24A to 24C show waveforms of parts of the color signal C, the luminance signal Y, and the composite video signal Sc generated from these signals.
The luminance signal Y, as shown in FIG. 24B, includes a horizontal synchronization signal HSYNC and a signal LMNS indicating the video luminance.
The color signal C, as shown in FIG. 24B, includes a color burst signal CBRST and a carrier color signal CARC.
The color burst signal CSRST becomes a phase reference of the carrier color signal CARC. A TV receiver or a video monitor uses the color burst signal CBRST to control the phase of the color sub carrier oscillator and reproduces three primary color signals R, G, B (red, green, blue) by the generated color sub carrier and the carrier color signal CARC.
Note that an actual color burst signal CBRST is comprised of 8 to 12 cycles of the color sub carrier frequency. FIGS. 24A to 24C show the concept thereof. This is different from the actual waveform.
As shown in FIG. 24C, the color burst signal CBRST of the color signal C is superimposed on a back porch PRCH of the luminance signal Y by the combining circuit 1, while the carrier color signal CARC is superimposed on the video luminance signal LMNS.
In the NTSC signal system, the frequency of the color burst signal CBRST and the carrier color signal CARC is 3.58 MHz. The luminance signal Y has a frequency band from the DC component of about 4.2 MHz.
Usually, the frequencies of the color burst signal CBRST and the color sub carrier for generating the carrier color signal CARC are selected so as not to allow interference between the luminance signal Y and the color signal C.
The NTSC signal system employs so-called “frequency interleaving” where the frequency of this color sub carrier is set to an odd number multiple of half of the horizontal scanning frequency.
Summarizing the problems to be solved by the invention, the above conventional video encoder cannot adjust the output level of the synthesized composite video signal Sc.
For this reason, when connecting a digital-to-analog converter (DAC) or driver or other device to the latter stage, the digital-to-analog converter must receive the fixed output level of the video encoder.
However, a digital-to-analog converter cannot exhibit its full performance when converting inputs near the maximum value and the minimum value, for example, in the case of a 10-bit input digital-to-analog converter, near 0 or 1023, to an analog format. Therefore, as mentioned above, when receiving a composite video signal Sc of a fixed level, a high precision digital-to-analog converter output may not be obtained.
Also, in a driver, there are some types that are supplied with 0V to +5V as the power supply voltage when trying to obtain an output of, for example, 0 to 1V. It is difficult to configure a system giving an output of 0V with a power supply voltage of 0V, so −5V is actually generated in the driver and −5V and +5V are supplied to an amplifier.
However, the −5V generated in the driver has a very small capacity, so if possible it is better that the signal be input in a region of +5V.
In this way, realization of a video encoder capable of adjusting the output level has been demanded.